1. Field of the Invention
The present invention relates generally to a method of fabricating a semiconductor device. More particularly, the present invention relates to a method of selective epitaxial growth (SEG) having improved selectivity.
2. Description of the Prior Art
Typical selective epitaxial growth is a useful technology applied to selectively grow a homogeneous or heterogeneous semiconductor layer on only an exposed semiconductor surface, and omitting epitaxial growth over a masked surface on an insulating layer, such as an oxide layer or a nitride layer.
Recently, various ways to obtain selective epitaxial growth of silicon or silicon germanium have been introduced in the art. Among them, an ultrahigh vacuum chemical vapor deposition (UHVCVD) technique has attracted considerable attention because it enables selective epitaxial growth at a low temperature, often at less than 800xc2x0 C. The UHVCVD technique employs mostly SiH4, Si2H6, GeH4 or Ge2H6 as a source gas, and Cl2 as a selectivity promoting gas.
On the other hand, the selective epitaxial growth technology is classified into either of two types, conventional growth and cyclical growth.
Conventional growth is a technique to grow an epitaxial silicon layer or an epitaxial silicon germanium layer by simultaneously supplying a source gas, such as SiH4, Si2H6, GeH4 and Ge2H6, and Cl2 gas into a reactor.
Cyclical growth is a technique to alternatively supply the source gas and Cl2 gas at alternating times in the manner shown in FIG. 1. Here, Cl2 gas serves to prevent the nucleus generation of polysilicon or silicon germanium on surfaces of the insulating layer due to either etching action or surface passivation.
Therefore, as illustrated in FIG. 2, the increase in a flow rate of Cl2 gas generally improves the selectivity of epitaxial growth.
Nevertheless, a high flow rate of Cl2 gas during selective epitaxial growth not only reduces the growth rate of the epitaxial layer, as depicted in FIG. 3, but also detrimentally affects the expected life span of the processing facilities. For at least the above reasons, there is a need to minimize the use of Cl2 gas.
After using a conventional method of selective epitaxial growth, a photograph of FIG. 4 shows an experimental result under conditions of Si2H6 source gas supplied with a flow rate of 10 sccm for about ten seconds in a first step and then Cl2 gas supplied with a flow rate of 2 sccm for about twelve seconds in a second step. Here the epitaxial growth is made on a semiconductor substrate having a nitride layer at a temperature of about 750xc2x0 C.
As shown in FIG. 4, a polysilicon layer is grown on the nitride layer; therefore the desired selectivity is not obtained. That is, since the nucleus of polysilicon or silicon germanium is generated first of all at a defective site on the surface of the insulating layer during a low temperature epitaxial growth less than 800xc2x0 C., the epitaxial growth is not selective.
It is therefore an object of the present invention to provide a method of selective epitaxial growth for a semiconductor device, capable of guaranteeing selectivity of the epitaxial growth and further preventing degradation of a gate oxide layer.
Another object of the present invention is to provide a method of selective epitaxial growth for a semiconductor device, allowing an improvement in productivity by increasing the growth rate of the epitaxial layer.
These and other objects in accordance with the present invention are attained by a method of selective epitaxial growth for a semiconductor device, the method comprising the steps of loading a semiconductor substrate into a reaction chamber, wherein a mask layer is selectively formed on the semiconductor substrate to define a first portion exposed beyond the mask layer and a second portion covered with the mask layer; supplying a source gas into the reaction chamber so that the source gas is adsorbed on the first portion of the semiconductor substrate and thus a semiconductor epitaxial layer is selectively formed on the first portion; supplying a selectivity promoting gas, including H2 gas, into the reaction chamber, whereby any nucleus of semiconductor material is removed from the surface of the mask layer of the semiconductor substrate and thus selectivity of the semiconductor epitaxial layer is improved; and sequentially repeating the steps of supplying the source gas and the selectivity promoting gas, whereby the semiconductor epitaxial layer is grown to a desired thickness.